A Rare Case of Waldenstrom Macroglobulinemia of the Rectosigmoid Colon

Waldenstrom macroglobulinemia is an uncommon mature B-cell lymphoma characterized by monoclonal immunoglobulin M protein in peripheral blood and lymphoplasmacytic cells in bone marrow and/or extramedullary sites. The gastrointestinal tract is a rare site of involvement. The diagnosis is based on clinicopathologic findings, although somatic mutations, such as MYD88, can aid in the diagnosis. We present a patient with irregular stools diagnosed with Waldenstrom macroglobulinemia involving the rectosigmoid colon by histopathology and immunohistochemistry on colonic biopsies, immunoglobulin M protein in serum, clonal plasma cells in bone marrow, and MYD88 mutation in colonic and bone marrow specimens.